1. Field of the Invention
The present invention relates to a semi-transmissive (trans-reflective) type liquid-crystal display (LCD) device and a method of fabricating the same. More particularly, the invention relates to a semi-transmissive type LCD device having pixel regions each of which is divided into a transmission region and a reflection region and a method of fabricating the device.
2. Description of the Related Art
In recent years, subsequent to the diffusion of portable information-processing equipment, the semi-transmissive type LCD device has been developed and used practically. With the device of this type, in a place where the ambient light is insufficient (i.e., a dark place), images are displayed by causing the light emitted from the backlight unit to pass through the liquid crystal layer. On the other hand, in a place where the ambient light is sufficient (i.e., a light place), images are displayed by causing the ambient light reflected by the internal reflector electrodes to pass through the liquid crystal layer.
With the semi-transmissive type LCD device having such the structure as above, the backlight is turned on and the images are displayed in the transmission mode, thereby raising the visibility in a dark place. The backlight is turned off and the images are displayed in the reflection mode, thereby reducing the power dissipation in a light place. Because of such the advantage, this device meets the two inconsistent demands to prolong the operable time and to reduce the weight.
The semi-transmissive type LCD device may have a structure that each of the pixel regions is divided into a transmission region and a reflection region. With this structure, the interlayer insulating film, which is formed to cover all the pixel regions, may be left in both the transmission region and the reflection region. Alternately, the said interlayer insulating film may be left in only the reflection region and removed in the transmission region. With the semi-transmissive type LCD device having this structure, a transmission electrode is placed in the transmission region and a reflection electrode is placed in the reflection region. In other words, each of the pixel electrodes is formed by the transmission electrode and the reflection electrode.
The Japanese Non-Examined Patent Publication No. 2004-101792 discloses a semi-transmissive type LCD device, which has the structure that the above-described interlayer insulating film is left only in the reflection region and removed in the transmission region (see FIGS. 1 to 3, paragraphs 0052 to 0058 and 0063 to 0067). The Thin-Film Transistor (TFT) array substrate used for this device is shown in FIG. 1, which is a partial cross-sectional view showing the pixel structure of the TFT array substrate. As shown in FIG. 1, the pixel region P corresponding to the said pixel is divided into a transmission region T and a reflection region R.
The prior-art TFT array substrate 121 shown in FIG. 1 comprises an insulative, transparent plate 101 on which gate electrodes 102 are formed. A gate insulating film 104 is formed on the plate 101 to cover the gate electrodes 102. On the gate insulating film 104, island-shaped semiconductor films 105 are formed to overlap with the corresponding gate electrodes 102. Pairs of island-shaped, heavily doped semiconductor films 106a and 106b are formed on the corresponding semiconductor films 105. Source electrodes 107 and drain electrodes 108 are overlapped with the corresponding pairs of semiconductor films 106a and 106b, respectively. The combination of the gate electrode 102, the gate insulating film 104, the semiconductor film 105, the pair of semiconductor films 106a and 106b, the source electrode 107 and the drain electrode 108 constitutes a TFT 115 as a switching element.
The TFTs 115 are covered with a passivation film 110. On the passivation film 110, transmission electrodes 113 are selectively formed. The transmission electrodes 113 are extended to not only the transmission regions T but also the reflection regions R. The transmission electrodes 113 are electrically connected to the corresponding drain electrodes 108 by way of the corresponding contact holes 110a. 
An interlayer insulating film 111 is selectively formed on the transmission electrodes 113 and the exposed parts of the passivation film 110 from the transmission electrodes 113. As shown in FIG. 1, the interlayer insulating film 111 does not exist in the transmission regions T and therefore, the transmission electrodes 113 are exposed from the interlayer insulating film 111 in the corresponding transmission regions T.
On the interlayer insulating film 111, reflection electrodes 114 are selectively formed. The reflection electrodes 114 are located in only the corresponding reflection regions R. The reflection electrodes 114 are electrically connected to the corresponding transmission electrodes 113 by way of the corresponding contact holes 112 at the overlapped positions with the corresponding contact holes 110a. In addition convex and concave shapes in other words, protrusions and depressions, are formed on the surface of the interlayer insulating film 111, which are provided to enhance the reflection effect of the reflection electrodes 14 formed on the film 111.
The TFT array substrate 121 shown in FIG. 1 for the prior-art semi-transmissive type LCD device is fabricated in the following way.
First, the gate electrodes 102 are formed on the insulative, transparent plate 101 and then, the gate insulating film 104 is formed on the plate 101 to cover the gate electrodes 102. Next, on the gate insulating film 104, the island-shaped semiconductor films 105 and the pairs of island-shaped, heavily doped semiconductor films 106a and 106b are successively formed to overlap with the corresponding gate electrodes 102 in this order. Thereafter, the source electrodes 107 and the drain electrodes 108 are overlapped with the semiconductor films 106a and 106b, respectively, thereby forming the TFTs 115. Then, the TFTs 115 are covered with the passivation film 110, and the transmission electrodes 113 are selectively formed on the passivation film 110 thus formed. At this time, the transmission electrodes 113 are contacted with the corresponding drain electrodes 108 by way of the corresponding contact holes 110a. All the process steps described so far can be carried out by known methods.
Subsequently, to form the interlayer insulating film 111, an insulative organic resin with photosensitivity is coated on the transmission electrodes 113 and the exposed parts of the passivation film 110 from the transmission electrodes 113, forming an organic resin film. Thereafter, the organic resin film thus formed is selectively exposed to predetermined light with a photomask. At this time, the pattern of the photomask is adjusted in such a way that the exposure energies are different from each other in the areas where the organic resin film is completely removed (which are the areas corresponding to the contact holes 112 and the transmission regions T) and in the areas where the convex and concave shapes are formed on the surface of the organic resin film (which are the areas corresponding to the reflection regions R except for those corresponding to the contact holes 112). The organic resin film thus exposed is then developed. As a result, the patterned organic resin film (i.e., the interlayer insulating film 111) whose thickness varies in accordance with the positions is obtained.
Concretely speaking, different photomasks are used for the areas where the organic resin film is completely removed and the areas where the convex and concave shapes (protrusions and depressions) are formed on the surface of the organic resin film. Alternately, a half-tone photomask (or a gray-tone photomask) on which a semi-transmissive film is partially formed is used. As a result, when the exposure energy for the areas where the organic resin film is completely removed is defined as 100%, the exposure energy for the areas where the convex and concave shapes are formed on the surface of the organic resin film is set at a value in the range from 10% to 50%.
Following this, the reflection electrodes 114 are selectively formed on the patterned organic resin film (i.e., the patterned interlayer insulating film 111) thus formed. In this way, the TFT array substrate 121 shown in FIG. 1 is obtained.
The Japanese Patent No. 3410656 discloses another semi-transmissive type LCD device, which has the structure that the above-described interlayer insulating film is left in not only the reflection region but also the transmission region (see FIGS. 1 to 4, paragraphs 0015 to 0016). The TFT array substrate used for this device is shown in FIG. 2, which is a partial cross-sectional view showing the pixel structure of the TFT array substrate. As shown in FIG. 2, the pixel region P corresponding to the said pixel is divided into a transmission region T and a reflection region R.
The prior-art TFT array substrate 221 shown in FIG. 2 comprises an insulative, transparent plate 201 on which gate electrodes 202 are formed. A gate insulating film 204 is formed on the plate 201 to cover the gate electrodes 202. On the gate insulating film 204, island-shaped semiconductor films 205 are formed to overlap with the corresponding gate electrodes 202. On the semiconductor films 205, pairs of island-shaped, heavily doped semiconductor contact films 206a and 206b are formed on the corresponding semiconductor films 205. The source electrodes 207 and the drain electrodes 208 are overlapped with the corresponding pairs, of semiconductor contact films 206a and 206b, respectively. The combination of the gate electrode 202, the gate insulating film 204, the semiconductor film 205, the pair of semiconductor contact films 206a and 206b, the source electrode 207 and the drain electrode 208 constitute a TFT 215 as a switching element.
With the TFT array substrate 221 of FIG. 2, no passivation film is provided, and an interlayer insulating film 211 is directly formed on the gate insulating film 204. As shown in FIG. 2, the interlayer insulating film 211 is present in not only the reflection regions R but also the transmission regions T. On the interlayer insulating film 211, transmission electrodes 213 are selectively formed. The reflection electrodes 214 are selectively formed on the corresponding transmission electrodes 213. The transmission electrodes 213, which are located in the corresponding transmission regions T, are extended to the corresponding reflection regions R and contacted with the corresponding drain electrodes 208 by way of the corresponding contact holes 212. The reflection electrodes 214, which are located in only the corresponding reflection regions R, are extended to the inside of the corresponding contact holes 212. The convex and concave shapes formed on the surface of the interlayer insulating film 211 exist in only the reflection regions R.
The TFT array substrate 221 shown in FIG. 2 for the prior-art semi-transmissive type LCD device is fabricated in the following way.
First, the gate electrodes 202 are formed on the insulative, transparent plate 201 and then, the gate insulating film 204 is formed on the plate 201 to cover the gate electrodes 202. Next, on the gate insulating film 204, the island-shaped semiconductor films 205 and island-shaped, heavily doped semiconductor contact films for the semiconductor contact films 206a and 206b are successively formed to overlap with the corresponding gate electrodes 202 in this order. Thereafter, the source electrodes 207 and the drain electrodes 208 are formed to overlap with the corresponding semiconductor films 205 and the semiconductor contact films 206a and 206b. Using the source and drain electrodes 207 and 208 as a mask, the island-shaped, heavily doped semiconductor contact films are selectively etched, thereby forming the pairs of semiconductor contact films 206a and 206b. In this way, the TFTs 215 are obtained. All the process steps described so far can be carried out by known methods.
Subsequently, an insulative organic resin with photosensitivity is coated on the TFTs 215 and the exposed parts of the gate insulating film 204 from the TFTs 215, thereby forming the interlayer insulating film 211. Then, the interlayer insulating film 211 thus formed is selectively exposed and developed. Thus, the contact holes 212 penetrating through the interlayer insulating film 211 to the corresponding drain electrodes 208 are formed and at the same time, the convex and concave shapes are formed on the surface of the interlayer insulating film 211 in the reflection regions R. In this way, the interlayer insulating film 211 having the pattern of FIG. 2 is obtained. Following this, the transmission electrodes 213 are selectively formed on the interlayer insulating film 211 thus patterned. The reflection electrodes 214 are selectively formed on the corresponding transmission electrodes 213 thus formed. In this way, the TFT array substrate 221 shown in FIG. 2 is fabricated.
The Japanese Non-Examined Patent Publication No. 2002-229049 discloses, like the previously-referred Japanese Patent No. 3410656, still another semi-transmissive type LCD device, which has the structure that the above-described interlayer insulating film is left in not only the reflection region but also the transmission region (see FIGS. 1 to 2, paragraphs 0010 to 0013). The pixel electrode of this device comprises a stacked structure including at least two layers of a transmission electrode layer and a semi-transmission electrode layer. This stacked structure is formed by a single patterning process of these two electrode layers using photoresist, resulting in the said pixel electrode.
According to the Publication No. 2002-229049, it is preferred that the semi-transmission electrode layer is formed by an Al, Al alloy, Ag or Ag alloy layer having a thickness of 20 nm or less. This is because the layers of these metals or alloys are high in reflectance and because the pixel electrode with a desired transmittance is obtainable. Moreover, it is preferred that the transmission electrode layer is formed by an indium tin oxide layer. This is because the indium tin oxide layer is low in resistance and transparent.
In this way, with the semi-transmissive type LCD device disclosed in the above-described Publication No. 2002-229049, the pixel electrode has the stacked structure comprising the transmission electrode layer and the semi-transmission layer. Therefore, the interlayer insulating film is left in not only the reflection regions but also the transmission regions. However, the pixel region P is not divided into the transmission region T and the reflection region R. As a result, this device is apparently different in structure from the device (see FIG. 2) disclosed in the previously-referred Patent No. 3410656.
With the prior-art LCD device disclosed in the Publication No. 2004-101792, as shown in FIG. 1, the interlayer insulating film 111 is left in only the reflection region R and does not exist in the transmission region T. Thus, by adjusting the thickness of the interlayer insulating film 111 left in the reflection region R, the optical path length of the light penetrating through the liquid crystal layer can be adjusted easily. Therefore, the optical path lengths of the light penetrating through the liquid crystal layer in the reflection region R and the transmission region T can be matched. This means that both the reflection characteristic and the transmission characteristic can be optimized easily.
However, the interlayer insulating film 111 does not exist in the transmission region T and thus, the pixel electrode (i.e., the transmission electrode 113) cannot be extended to overlap with the wiring lines. As a result, a problem that the aperture ratio decreases occurs.
With the prior-art LCD device disclosed in the Patent No. 3410656, as shown in FIG. 2, the interlayer insulating film 211 is left in not only the reflection region R but also the transmission region T. Thus, contrary to the prior-art device of FIG. 1, the pixel electrode (i.e., the transmission electrode 213) can be extended to overlap with the wiring lines and as a result, the aperture ratio can be increased. On the other hand, however, the interlayer insulating film 211 exists in the whole pixel region P and therefore, it is not easy to form a desired level difference between the reflection region R and the transmission region T.
As a result, there is a problem that optimization of both the reflection characteristic (i.e., the reflection mode) and the transmission characteristic (i.e., the transmission mode) is difficult by matching the optical path lengths of the light penetrating through the liquid crystal layer in the reflection region R and the transmission region T.
With the prior-art LCD device disclosed in the Publication No. 2002-229049, each pixel region is not divided into the transmission region and the reflection region. Thus, similar to the prior-art device shown in FIG. 2, there is a problem that optimization of both the reflection characteristic (i.e., the reflection mode) and the transmission characteristic (i.e., the transmission mode) is not easy.